Heat exchangers



March 3 70 r. D. PATTEN ET AL HEAT .EXCHANGERS 4 Sheet t 2 Filed April 10, 1968 March 3, 1970 1-. D. PATTEN ETAL- 3,498,372

HEAT. EXCHANGERS 4 Sheets-Sheet 3 Filed April 10, 1968 March 3, 1970 1'. n. PATTEN ET AL 3,498,372

Y HEAT, EXCHARGERS Filed April 10, 1968 '4 Sheets-Sheet 4 FIG. 70. 58

United States Patent 3,498,372 HEAT EXCHANGERS Thomas Diery Patten, Edinburgh, Scotland, and John Leslie Collins, Kings Heath, Birmingham, England, assignors to National Research Development Corporation, London, England, a British corporation Filed Apr. 10, 1968, Ser. No. 720,112 Claims priority, application Great Britain, Apr. 14, 1967, 17,365/ 67 Int. Cl. F28f 3/00 US. Cl. 165-166 8 Claims ABSTRACT OF THE DISCLOSURE A heat exchanger matrix made of superimposed plates each embossed with a regular pattern of pyramid-shaped projections interspersed with indentations. These are called Type I plates. The plates are in pairs with the projections of one plate of the pair (the deep plate) being deeper than those of the other (the shallow plate). Viewed from the side of, and looking down upon, the projections, the shallow plate lies beneath the deep plate with the crests of the projections aligned so that the crests of the shallow plate are convex towards the undersides of the projections of the deep plates. The next pair below 13 displaced laterally of the direction of fluid flow between the plates by half the crest-to-crest pitch of the projections so that the two pairs meet with the space heneath each projection of a shallow plate of the pair opposite to an indentation of the deep plate of the pair below. Thus the cross-sectional area of the fluid flow path between the plates of a pair is less than that between adjacent pairs of plates. In a first form of matrix the projections of the plates are interspersed with indentations of complementary shape so that each base side of a projec tion is coincident with a base side of an adjacent indentation. The plates of a pair are identical except for the heights and depths of the crests of the projections and indentations from a common media plane.

A second modified version of this form of matrix has a third (shallow) plate above the deep plate of each pair. The two shallow plates are in register crest-for-crest of their projections and define twin fluid flow channels separated by the deep plate whose depth determines the combined cross-sectional area of the twin channels, and which provides secondary heat exchange surfaces within the twin channels.

The other fluid flow channel is formed between the lower shallow plate of one pair and the third shallow plate of another pair these two shallow plates meeting crest-to-crest as before but with smaller cross-sectional area for the fluid flow path than is provided when a shallow plate and a deep plate define the channel.

A third form of matrix uses a different type of plate, called a Type II plate for the shallow plate of each pair. It has adjacent pyramidal projections, upstanding from a common base plane, and arranged in two sets of rows at right angles to one another, with crossed furrows between them. The pitch of the furrows and of the parallel rows of projection crests is equal to the diagonal distance across the base of a projection (or of an indentation) of the deep Type I plates of the pairs. The shallow Type II plates have the said rows of the crests of projections aligned with diagonal rows of crests of projections of the deep Type I plates and the crests of the projections of the two plates of the pair are in register as in the first form of matrix.

The first and second forms of matrix provide fluid flow paths which do not vary in cross-sectional area along 3,498,372 Patented Mar. 3, 1970 the length of the fluid flow path. With the third form there is a certain amount of variation of these cross-sectional areas which causes certain energy losses but as Type II plates will, in general, be cheaper to make the third form of matrix may be preferred where cost is more important than efficiency.

The present invention relates to heat exchangers and is concerned with heat exchangers in which the flow channels for the heat exchange fluids are defined by the surfaces of a matrix of plates stacked one against the other.

It has been proposed in British patent specification No. 901,914 to construct a heat exchanger comprising a plurality of plates stacked one against another in which the plates consist of thin sheets on which are embossed a pattern of alternating four-sided pyramidal projections and indentations, the said pattern being such that each base side of a pyramidal projection is common with a base side of an adjacent indentation and each base side of a pyramidal indentation is common with a base side of an adjacent projection so that the crests of the said projections and indentations lie on straight lines in two directions substantially at right angles to one another, and means are provided for stacking the plates so that the crests of corresponding projections and indentations on adjacent plates are substantially in contact whereby the cross-sectional flow area between the embossed area surfaces of adjacent plates, taken in either of the aforementioned directions on which the crests of the projections and indentations lie, is constant. By blocking up the flow channels between suitable parts of the edges of the plates, the heat exchanger can be arranged for operation with co-currrent, counter-current or cross-current flow of the heat exchange fluids.

The flow channels for the heat exchange fluids in this previously proposed heat exchanger are all of substantially identical characteristics and, in consequence, it is not always possible to achieve the desired heat exchange in an eflicient manner.

According to one aspect of this invention, a heat exchange matrix for a heat exchanger includes a plurality of pairs of plates stacked one against another, each plate being provided with a regular pattern of projections and the plates in each pair being so arranged that the crests of the projections on one plate penetrate within the hollow spaces of the other plate, enclosed within the pro jections on the other plate so as not to fill those spaces.

The projections of each plate may be separated from each other and/ or defined by a regular pattern of indentation in each plate. The said space between the crests of the projections of the plates in each pair provides a flowchannel for one heat exchange fluid. A flow channel for another heat exchange fluid is provided between adjacent pairs of plates.

The heat exchange matrix according to this aspect of the invention provides flow channels of diflerent characteristics for each of the heat exchange fluids, and by an appropriate choice of the dimensions of the projections on the plates in each pair of plates, the flow channels for each heat exchange fluid may have characteristics suitable for the duties to be undertaken by the heat exchanger.

The projections may be of any suitable shape-e.g.

V pyramidal with a square or triangular base.

A heat exchanger in accordance with the present invention in which all of the plates have regularly spaced projections and indentations of four-sided pyramidal form with the base side of a projection common with the base side of an indentation, may be regarded as a modification of the heat exchanger described in British patent specification No. 901,914 in which alternate plates have been displaced in their respective planes a distance equal to half the distance between adjacent crests of the projections.

Preferably, the projections and indentations on one plate of each pair of plates upstand from the mean plane of the plate a different distance to the projections and indentations of the other plate of the pair; thus, in the instance where the projections and indentations are of pyramidal shape, the projections and indentations of the plates in each pair may have differing apex angles and the projections and indentations of one of the plates in each pair may be truncated. Hereafter, for convenience of reference, a plate in which the apex angles are relatively large will be referred to as a shallow plate and a plate in which the apex angles are relatively small, will be called a deep plate.

Where each of the plates in each pair of plates are of substantially similar shape, the cross-sectional area of the fiow channels for each heat exchange fluid can be arranged to be substantially constant in all cross-sectional planes perpendicular to the plates. It is thought that when the cross-sectional area of a flow channel is constant, power losses due to compressions and rarefactions of the fluid passing therethrough are minimised, during operation.

In one arrangement of heat exchanger in accordance with the invention, shallow plates are stacked alternately with deep plates whereby to provide a flow channel for a respective fluid between each pair of plates. In this arrangement each plate separates a flow channel for one fluid from a flow channel for another fluid, so that, in operation, heat exchange takes place through all parts of the plates which are contacted by a heat exchange fluid.

In another arrangement of heat exchanger in accordance with the invention, a pair of shallow plates is stacked against each of the deep plates, the first-named pair of plates being arranged with a projection of one plate disposed opposite an indentation of the other plate whereby to form a flow channel for one fluid between the said pair of shallow plates and a flow channel for another fluid outwardly of the said pair of plates. In this arrangement the deep plates separate adjacent pairs of the shallow plates to provide a flow channel for the said other fluid between the adjacent pairs of the shallow plates. The heat exchange takes place only across the shallow plates while the deep plates provide so-called secondary heat exchange surfaces. The flow channels, according to this arrangement are of substantially uniform cross-sectional area as in the previous arrangement.

According to another form of the invention, in each pair of plates, at least one of the plates may be formed only with regularly spaced projections and adjacent projections may have their base sides adjacent or in common. In one kind of matrix in accordance with this form, each pair of plates comprises one plate having only projections formed thereon, and the other plate is formed with regularly-spaced projections and indentations. This kind of matrix tends to be somewhat cheaper than the other described forms of matrix in accordance with the invention, but provides flow channels whose areas vary from cross section to cross-section; it is believed that this variation in the area of the flow channels may. lead to power losses in the heat exchange fluids during operation.

The invention will now be described by way of a number of non-limitative examples and with reference to the accompanying drawings, in which:

FIGURE 1 is a perspective view of part of a plate for use in a heat exchanger in accordance with the invention,

FIGURE 2 is a transverse cross-sectional view taken in the plane IIII of the plate of FIGURE 1,

FIGURE 3 is a view of part of one form of heat exchange matrix in accordance with the invention taken in a transverse plane through the matrix corresponding with the plane IIIIII of FIGURE 1,

FIGURE 4 is another cross-sectional view of part of the matrix shown in FIGURE 3 but taken in a plane corresponding with the plane 11-11 of FIGURE 1,

FIGURE 5 depicts, in a cross-sectional view corresponding with that of FIGURE 3, another form of part of a matrix in accordance with the invention,

FIGURE 6 shows part of the matrix of FIGURE 5 in a cross-sectional view corresponding with that of FIG- URE 4,

FIGURE 7 is a perspective View of part of another plate for use in a matrix in accordance with the invention, an

FIGURE 8 is a cross-sectional view of part of the matrix shown in FIGURE 7.

FIGURE 9 is an exploded perspective view of yet another form of matrix for use in a heat exchanger according to the invention, and

FIGURES 10, 11 and 12 are cross-sectional views, sectioned in various planes, of a matrix according to FIGURE 9.

Referring first to FIGURE 1, the plate 1 is in the form of a thin sheet of material having suitable heat conducting properties and resistance to corrosion by the fluids with which it is to be in contact during operation. For duties at low temperatures and where the heat exchange fluids are relatively inactive, chemically and physically, the plate 1 may be of sheet metal or of ceramic. For more severe duties, more particularly at elevated temperatures, it may be preferred to employ plates of thermal shock-resistant ceramics, such as those available from the Corning Glass Works Corporation of the United States of America or the Minnesota Mining and Manufacturing Corporation. On the plate 1 is formed a pattern of regularly spaced projections of four-sided substantially pyramidal form and having crests 2, alternating with regularly spaced indentations also of four-sided substantially pyramidal form and having crests 3. The crests 2 have been ringed in FIGURE 1 for ease of identification. The projections and indentations are so arranged relative to each other that each base side, e.g. side 4, of the projections is also the base side of the indentations, and vice-versa, and the crests 2 and 3 of the projections and indentations of the plate 1 lie along two sets of straight lines e.g. 5 and 6, substantially at right angles to each other.

The crests 2 and 3 are thus aligned in a pattern of rectangles whose side are parallel to the straight lines 5 and 6.

It will be seen that cross-sectional view of the plate 1 in transverse planes, such as IIIIII, which include the crests 2 of the projections and the crests 3 of the indentations have the same configuration as that depicted at the front edge 7 (as viewed) of the plate 1, that is to say, a configuration resembling a succession of VS. Crosssectional views of the plate 1 in transverse planes, such as IIII, which intersect the base sides of the projections and indentations have a linear configuration, as depicted in FIGURE 2. This type of plate will hereinafter be referred to as a Type 1 plate to distinguish it from the type of plate illustrated in FIG. 7, which will be referred to as a Type II plate.

Referring now to FIGURE 3, the section of the matrix 15 is seen to comprise pairs of adjacent Type I plates 8/9, 10/11, 12/13 having the general form described in relation to FIGURES 1 and 2. The spacing of the projections and indentations on all of the plates is identical but plates 8, 10 and 12 are deep plates and plates 9, 11 and 13 are shallow plates. The plates in each pair of plates are so arranged that the crest such as 14 of every projection of every shallow plate such as 9 is opposite and spaced apart from the under side of the crest such as 15 of a projection of every deep plate such as 8; that is to say, in each pair of plates 8/9, 10/11, 12/13, the crest such as 14, of every projection of the shallow plate of the pair is in the same straight line normal to the mean plane of the plates as the crest such as 1-5 of a neighbouring projection of the deep plate of the pair and spaced apart therefrom to provide a flow channel such as 16 therebetween for a heat exchange fluid. The flow channels such as 16 are hatched with lines sloping from top right to bottom left for the sake of clarity.

Similarly, in each pair of plates, the crest such as 17, of every indentation of a shallow plate such as 9 is opposite the crest such as 18, of an indentation of a deep plate such as 8. The crests such as 17, 18 of the opposed indentations of the plates such as 8, 9 in each pair are in substantial contact and locate the plates such as 8, 9 relative to each other. In the embodiment of FIGURE 3, the spacing or separation of neighbouring projections such as 14, of each pair of plates such as 8, 9 is adjusted by suitable choice of the relative apex angles of the crests of the projections of the shallow plates and the deep plates of each pair. The separation of neighbouring projections of the plates of each pair may be achieved in other ways. For example, spacing members, such as washers (not shown), may be disposed between the crests such as 14 and 15 of neighbouring projections or the crests such as 17, 18 of neighbouring indentations of the paired plates such as 8, 9.

A flow channel 19 for a second heat exchange fluid is provided in the space between adjacent pairs of plates by the abutment between the crests such as 20 of the projections of one plate the deep plate such as 10 of a pair of plates such as 10, 11 and the crests such as 17 of the indentations of a shallow plate such as 9 of a neighbouring pair of plates such as 8, 9.

The flow channels 16 and 19 each have a constant cross-sectional area, although a varying shape, from crosssection to cross-section.

Flow channels corresponding to flow channel 16, for the other pairs of plates are similarly hatched and numbered 21 and 22 respectively. Flow channels such as 19 are all similarly hatched with lines slanting from top left to bottom right and those between plate pairs 10, 11 and 12, 13 are numbered 23. This is thought to minimise energy losses in the heat exchange fluids.

In the cross-section of FIGURE 4 passing through the base-sides of the projections and indentations of the plates 8, 9, 10, 11, 12 and 13 of FIGURE 3 (and thus corresponding with the plane through line IIII of FIGURE 1 perpendicular to the mean plane of each of the plates), the flow channels 16, 19, 21, 22 and 23 are parallel-sided, but the cross-sectional areas are the same as in the cross-section of FIGURE 3. It will be seen that heat exchange can take place through all parts of each plate and this form of the matrix is compact and light in weight when designed for duties in which the heat transfer coefficients between the heat exchange surfaces and the fluids in contact with them are all of the same order of magnitude. It will be appreciated that the cross-sectional area of each of the pairs of flow channels such as 16 and 19 can be chosen to suit the duties of the matrix by forming the matrix from shallow plates and deep plates having projections and indentations of suitable relative apex angles and heights from the mean plane of each plate.

Fluid can enter or leave the flow channels 16, 21, 22 or 19, 23 at some of the edges of the matrix, these edges conveniently having the form depicted in FIGURE 4 so that they can be connected to suitable manifolding for separating the flows or blocked by sealing members (not shown) trapped at these edges of the matrix between the plates.

In the case of cross flow heat exchange being required 6 alternate channels (e.g. 16, 21, 22) can be blocked along one pair of parallel edges encased in manifolds for one of the flows whilst the intermediate channels (e.g. 19, 23) can be blocked along another pair of parallel edges also enclosed in manifolds for the other of the flows.

Referring now to FIGURE 5, thematrix, generally designated 24, may be considered to be the same in basic form as the matrix of FIGURES 3 and 4, having pairs of adjacent plates stacked one against another having the same reference numerals as corresponding plates in FIGS. 3 and 4 and pair against pair in the same formation as is shown in FIGS. 3 and 4, but with each pair having a third plate, 25 for pair 8/ 9, 26 for pair 10/11, 27 for pair 12/13, at the top of the pair as seen in FIG. 5. The third plates 25, 26 and 27 are shallow plates similar to plates 9, 11 and 13 and each of them lies over the upper deep plate of the associated pair of plates so that the crests of the projections (such as 15) penetrate into the undersides of the crests of the projections (such as 28) of the third plates (such as 25).

The effect of this is to provide two shallow plates, such as 9, 26 or 11, 27, stacked with the crests such as 14 of the projections of plate 9 aligned with the crests, such as 29, of the indentations of plate 26 and similarly for plates 11 and 27 and other corresponding plates of a complete matrix of this form. These pairs of shallow 1 plates such as 9, 26 and 11, 27, enclose flow channels 30 and 31 (hatched in FIGURE 5, for the sake of clarity with lines slanting from top right to bottom left) for one heat exchange fluid and the space between the lower shallow plate of, and the shallow third plate associated with, each pair (e.g. between plates 25 and 9, 26 and 11, 27 and 13) provides pairs of flow channels for the other heat exchange fluid, the channels of each pair (e.g. 32, 33 for plates 25 and 9; 34, for plates 26 and 11 and 36, 37 for plates 27 and 13) being sub-divided and their sizes being determined by the crest angles of the projections and indentations of the upper deck plate of each pair (e.g. 8, 10 and 12 respectively) which act as secondary heat exchange surfaces.

The paired channels 32 and 33, 34 and 35 and 36 and 37 are hatched with lines slanting from top left to bottom right. A matrix of the form 24 of FIG. 5 provides advantages in compactness and weight when designed for duties in which the heat transfer coefiicient between the fluid which is to flow in the channels 21 and the unpaired plates defining the channels 32 and 33, 34 and 35 and 36 and 37, is substantially less than the heat transfer coefficient between the fluid which is to flow in the channels 30 and 31. For such duties, the matrix 24 would be lighter in weight and would occupy a smaller volume than the matrix of FIGURE 3.

As will be appreciated from FIGURE 6, in cross-sectional planes through the common base-sides of the projections and indentations, all the flow channels have parallel boundaries, but the cross-sectional area of the channels is substantially the same as in the plane of FIGURE 5. The heights from the mean plane of the plates, of the projections and indentations and their apex angles, are so chosen that the cross-sectional areas of the various flow channels are suitable for the duties of the matrix 24.

FIGURE 7 shows a plate 38 of a somewhat different type from that shown in FIG. 1 and this will be called a Type II plate.

A Type II plate is formed with a pattern of regularly spaced projections (such as 39, 40), of four-sided pyramidal form which define a pattern of intersecting furrowshaped indentations (such as 41, 42) between adjacent projections. The base side (e.g. 43) of each projection (e.g. 44) is common with the base side of the adjacent projection (e.g. 45) and all the base sides lie substantially in a common base plane and provide the crests of a grid of furrow-shaped indentations.

FIGURE 8 illustrates the cross-sectional profile of Type II plate 38 taken in the plane of line VIIIVIII through the crests of the pyramidal indentations. The cross-sectional profile of the plate 38 in the plane of line 11-11 through the base sides of the pyramidal projections i.e. through the bottoms of the furrow-shaped indentations is linear, substantially as depicted in the cross-section of FIGURE 2.

FIGURE 9 is an exploded perspective drawing of three plates 46, 47 and 48 of a matrix using Type I plates and Type II plates alternately. Plates 46 and 48 are deep Type I plates and plate 47 is a shallow Type II plate. Plates 46 and 47 form a pair and plate 48 is the upper plate of a second pair which is displaced in two orthogonal directions by a distance equal to say the length of the base side (e.g. 43) of one of the projections of the Type I plates.

The bottoms (e.g. 42 and 49) of the furrow-shaped indentations of the Type II plates such as 47 are spaced apart by a distance measured in directions parallel to the base side (e.g. 50) of a projection (e.g. 40), which is equal to the diagonal (e.g. 51) across the base of a projection (e.g. 52) of a Type I plate (such as 46 or 47). The plates, such as 46, 47, of a pair are stacked together with the bottoms of the furrow-shaped depressions (such as 42 and 41) of the lower Type II plate (such as 47) registering with and in contact with diagonal rows (such as are indicated by chain-dotted lines 53 and 54 in FIGS. 1 and 9) of crests of depressions of the overlying Type I plate (such as 46).

The underside ridges corresponding to the bottoms of the said furrow-shaped depressions of the Type II plate (such as 47), of each pairs register with and make contact with diagonal rows (such as 55, 56 in FIGS. 1 and 9) of crests of projections of the upper Type I plate of the pair below.

In FIG. 9, dotted lines A-A, B-B, C-C, DD, E-E, and F-F indicate the intersection of the plates with various planes normal to the median planes of the plates and normal to the direction of flow of the heat exchange fluids which direction is shown by arrow 57 in FIG. 9. FIG. shows the cross-section of the part-matrix of FIG. 9, in the planes B-B and E-E; FIG. 11 in the planes A--A, C-C and FF; FIG. 12 in the plane DD. It will be seen that, in the DD section of FIG. 12, there 'are two flow paths which resemble in shape the flow paths 16 and 19 of FIG. 3 and these numerals and hatching as in FIG. 3 are adopted for FIGS. 10, 11 and 12.

It will be evident from a study of FIG. 9 that the same sequence of cross-sections of flow paths as is shown in FIGS. 10, 11 and 12 would be obtained if the fluid flow was in the direction indicated by arrow 58 in FIG. 9. It therefore follows that this form of matrix is adapted for cross-flow heat exchange.

The heat exchange matrices of the form shown in FIGS. 9 to 12 provide flow channels for the heat exchange fluids whose cross-sectional area varies from section to section, which is disadvantageous from the point of view of power losses in the heat exchange fluids. The variation is not extreme however and the losses may be acceptable for some applications.

Matrices made with Type I plates only, require two forms of plate, one deep and one shallow. The tools for making Type I plates are costly and two sets of them are required. The tool for making a Type II plate could be a slab of metal with crossed grooves cut into it by simple milling or planning operations. This tool should be much cheaper than the tool for a Type I plate. Therefore a matrix made up of deep Type I plates from one expensive tool and shallow Type II plates, from one cheap tool should be cheaper to produce than a matrix of two different Type I plates, except for very long production runs. This compensates to some extent for the increased losses due to variations of cross-sectional areas of the flow paths along the line of flow.

Heat exchanger matrices in accordance with the invention may take forms other than those described above;

for example the profiles of the projections and/or indentations on at least some of the plates may be curved in cross-section in planes parallel to the plane of the plate.

In a convenient modified form of matrix in accordance with the invention, the projections and/or indentations may have fiat-topped crests. Where the plates constituting the matrix are relatively thick, it may be easier to press out flat topped projections and indentation-s on the plates than fully pyramidal projections and indentations. A matrix modified in this manner may be made particularly rigid where metal plates are employed by spot welding a small number of the fiat-topped projections :and/or indentations of one plate to the contiguous projections or indentations of the neighbouring plate. The flat tops facilitate such spot welding.

The matrices may be sealed at the edges of the flow channels by fianging together the appropriate edges of the plates.

The heat exchanger matrices in accordance with the invention may be arranged to provide heat exchange between liquids, gases or gases and liquids, flowing in concurrent, counter-current or cross-current according to the manner in which the edges of the matrices are sealed.

In the foregoing description and in the claims which follow the expressions projection and indentation refer to the appearance of a plate as seen from one side, a projection having its crest nearer to the viewer than its base and an indentation having its base nearer to the viewer than its crest. When the same plate is viewed from the other side, what was formerly a projection :appears as an indentation and what was formerly an indentation appears as a projection. Thus a description of a matrix such as that illustrated in FIG. 3 which is appropriate for the matrix as seen from the top in FIG. 3, is not appropriate if the matrix is turned upside down, (through this inversion does not alter the nature of the matrix), unless the terms projection and indentation are interchanged throughout.

Tcllie description and claims are to be read with this in min We claim:

1. A matrix for a heat exchanger comprising a stack of plates placed one above the other, the spaces between ad acent plates providing fluid flow channels characterised in that the plates have formed upon them a pattern conslstm g, as seen from one side of the plate, of pyramidal pro ections interspersed with depressions, each such pro ection, when seen from the other side of the plate, defin ng a projection, the plates being stacked together in pairs, at least one plate of each pair having pyramidal pro ections extending in one direction from a median plane, and depressions extending in the other direction from the said median plane, each side of a projection bemg coplanar with a side of an adjacent depression to form together a common quadilateral surface divided by the said median plane into two triangular sections, one pertaining to a projection and the other to a depression, one plate of each pair called the deep plate, having relatively deep projections and depressions and the other plate of the pair, called the shallow plate, having relatively shallow projections and depressions, the shallow plate having such projections in register with and penetrating within the depressions defined, when seen from the other side of the said other plate, by the deep projections of the deep plate of the pair, so as to form a flow channel of relatively small cross-section between the plates of the pair, pairs of plates being stacked together so that the crests of projections, as seen from the said other side of a plate of one pair are in register with projections, as seen from the said one side of an adjacent plate of an adjacent pair of plates, so as to form, between pairs of plates, at flow channel of relatively large crosssection.

2. A matrix according to claim 1 characterised in that both plates of a pair have the same general form of pattern formed upon them and differ one from another solely in that the projections and depressions of one plate of a pair extend by a relatively small amount from the said median plane and the projections and depressions of the other plate of a pair extend by a relatively large amount from the said median plane.

3. A matrix according to claim 1 characterised in that the shallow plate of a pair has formed upon it, as seen from one side of the plate, an array of pyramidal projections in two rows at right angles to one another with the bases of the triangular sides of adjacent projections lying on a common line so as to form troughs at right angles to one another between the rows of projections, the dimensions of the projections of the shallow plate being such and the plates of a pair being assembled together in such a manner that each said common line of a shallow plate lies in a plane which intersects the crests of the projections but not the bottoms of the depressions of the deep plate of the pair, pairs of plates being assembled together with the projections of the deep plate of one air in register with the depressions of the deep plate of an adjacent pair as seen from the same side of both the said deep plates.

4. A matrix as claimed in claim 2 characterised in that each pair of plates is augmented by a third shallow plate stacked against the deep plate of the pair, the two shallow plates of the augmented pair being in register crest for crest, of their projections, as seen from the same side of both plates, in planes at right angles to the median planes of the plates, such augmented pairs of plates being stacked together with the projections of the deep plate of one augmented pair in register with the depressions of the deep plate of the other augmented pair as seen from the same side of both the said deep plates.

5. A matrix as claimed in claim 1 characterised in that the crests of projections are truncated.

6. A matrix as claimed in claim 1 characterised in that the projections are interspersed with indentations of complementary shape to the projections and in which the crests of projections and of indentations are truncated.

7. A matrix according to claim 6 characterised in that contacting crests of projections and indentations of adjacent plates are spot-welded together.

8. A matrix according to claim 7 in which the projections and indentations have curved profiles.

References Cited UNITED STATES PATENTS 1,968,351 7/1934 Pieper -166 X 2,064,931 12/1936 Lysholm 16510 X 2,462,421 2/1949 Pitt 165166 X ROBERT A. OLEARY, Primary Examiner THEOPHIL W. STREULE, Assistant Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 r 372 Dated March 3, 1970 Inventor(s) Patten, Thomas Diery and Collins, John Leslie It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 48, before "projection", insert --one Column 8, line 49, after "a" and before the word "projection", please insert --depression, and each such one side depression, when seen from the other side of the plate, defining a Signed and sealed this 11th day of May 1971.

(SEAL) Attest:

EDWARD M.F'LETGHER, JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents FORM PO-IOSO (10-69) UscMM Dc eoznbpsg i: us covuunzm PIIIIIING omc: nu o-au-su 

